This spatial data layer is a cell-based Raster model characterizing the contribution of phosphorus (P) to streams from weathering and erosion of surficial geologic materials for the conterminous United States, and has been created as part of the SPARROW (Spatially Referenced Regressions on Watershed Attributes) water-quality model implementation. SPARROW provides estimates of total nitrogen and phosphorus load in surface waters from point and non-point sources. This data set represents only potential for P contributions, and does not include N. The characterization of contribution of P from geologic materials will help separate the effects of natural or background sources from anthropogenic sources of P, such as from municipal wastewater or agricultural practices. The potential of a watershed to contribute P from naturally occurring geologic materials in the watershed to the adjacent receiving stream is characterized using geochemical data from bed-sediment samples (USGS National Geochemical Survey, National Uranium Resource Evaluation (NURE), Smith 1979) collected from first-order streams in relatively undisturbed watersheds.
Bed sediment samples were collected from 1975 through 1980. The bed-sediment phosphorus concentrations (BSPconc) in this data set represent BSPconc in the absence of anthropogenic sources and therefore spatial variation in BSPconc can be used to indicate spatial variation in the potential to contribute naturally occurring P from the geologic materials in the watershed to the stream. A continuous mapped distribution of BSPconc in headwater streams for the conterminous U.S. was prepared by summarizing BSPconc from 50,740 bed-sediment samples by geologic unit using regional-scale geologic map units (GMU's). The GMU's delineate areas with similar lithochemistry based on geologic age as well as ecoregion units. GMU’s were created by combining surficial geology (Hunt, Charles D., 1979) with Level IV Ecoregion units (U.S. Environmental Protection Agency 2011). This data set represents the mean BSPconc value for each GMU. The spatial pattern of BSPconc in headwater streams is offered as a data layer, representing best available information at the regional scale, with the caveat that its performance as a surrogate for contribution of P from watershed geologic materials to in-stream levels of P is weakened by not accounting for variability across headwater streams in the rates of net deposition and processing of P in the streambed.